ch19_arom

CHAPTER 19 ~ CURRENT AND RESISTANCE!

KEY VARIABLES:
I: Current (A)
$\displaystyle \Delta{Q}$ : Charge (C)
$\displaystyle \Delta{t}$ : time (s)
R: Resistance (ohms)
$\displaystyle \Delta{V}$ : Potential Difference (V)
P: Power (W)
$\displaystyle \Delta{PE}$ : change in potential energy (J)

KEY EQUATIONS:
$\displaystyle I=\frac{\Delta{Q}}{\Delta{t}}$
$\displaystyle R=\frac{\Delta{V}}{I}$
$\displaystyle \Delta{V}=IR$
$\displaystyle \frac{\Delta{V}}{I}=constant$
$\displaystyle P=\frac{W}{\Delta{t}}=\frac{\Delta{PE}}{\Delta{t}}=\frac{q\Delta{V}}{\Delta{t}}$
$\displaystyle I=\frac{q}{\Delta{t}}$
$\displaystyle P=I\Delta{V}$
$\displaystyle P=I^2R$
$\displaystyle P=\frac{\Delta{V}^2}{R}$

KEY CONCEPTS:
SECTION 19-1

• Electric current (I) is the rate at which positive charges move through a conductor past a fixed point.
• Current is measured in amperes (A) which are a fundamental SI unit.
• Regard current as being the movement of positive charges, but in common conductors such as metals it is actually the negatively charged electrons that are moving. In such a case the current is considered to be positive charges moving in the opposite direction.

Pg. 696 from Holt Pysics by Serway and Faughn

• Batteries convert chemical energy into electrical energy and generators convert mechanical energy into electrical energy.
• Batteries always produce a fixed potential difference between their terminals (direct current), generators can either produce the same type of potential difference or one that reverses itself many times a second (alternating current).

SECTION 19-2

• The current through some circuit depends on the potential difference in the current source, a larger potential difference produces a larger current.
• The opposition to the flow of current through a conductor is called resistance (R).
• Resistance is measured in ohms.
• Ohms Law: the resistance is constant over a wide range of applied potential differences.
• Materials that follow this relationship are said to be ohmic, others (such as semiconductors) are non-ohmic. Materials may be ohmic at one temperature and non-ohmic at another.
• R depends on length, cross-section, material, and temperature. Longer conductors, smaller cross-sections, and higher temperatures all increase the resistance of a conductor and vice versa, replacing the material making up the conductor with another material will also change the resistance.
• The resistance of the human body is about 500 000 ohms if the skin is dry and can drop to 100 ohms if the skin is wet.
• Some materials have zero resistance below a certain critical temperature, these materials are known as superconductors.

SECTION 19-3

• Just as power was studied previously as the rate of conversion of mechanical energy it is also the rate at which electrical energy is transferred.
• When electric utilities sell energy they use the derived unit kilowatt-hour.
• The power lost to resistance in a wire is proportional to the square of the current.
• Transformers are used to reduce potential difference to 120V close to the point where it will be put to use.

The organization and overall structure match the 2002 edition of Holt Physics by Serway and Faughn and many of the expressions and the ideas come from there as well. Much of wording and ideas also come from Honors Physics Review Notes 2008-2009 by Strong.

Citations:

Faughn, Jerry S. and Raymond A. Serway. Holt Physics. New York, Holt. 2002.
Strong, Tom. Course notes. Honors Physics. Dept. of Science. Mt Lebanon High School. May and June 2009.